The availability of high performance optical amplifiers such as erbium-doped fiber amplifiers (EDFA's) has renewed interest in the use of wavelength division multiplexing (WDM) for optical transmission systems. In a WDM transmission system, two or more optical data carrying channels, each defined by a different carrier wavelength, are combined onto a common path for transmission to a remote receiver. The carrier wavelengths are sufficiently separated so that they do not overlap in the frequency domain. The multiplexed channels are demultiplexed at the receiver in the electrical or optical domain. Demultiplexing in the optical domain requires using frequency-selective components such as optical gratings or bandpass filters. Typically, in a long-haul optical fiber system, the set of wavelength channels would be amplified simultaneously in an optical amplifier based repeater.
One class of optical amplifiers are rare-earth doped optical amplifiers, which use rare-earth ions as a gain medium. The ions are doped in the fiber core and pumped optically to provide gain. The silica fiber core serves as the host medium for the ions. While many different rare-earth ions such as neodymium, praseodymium, ytterbium etc. can be used to provide gain in different portions of the spectrum, erbium-doped fiber amplifiers (EDFAs) have proven to be particularly attractive because they are operable in the spectral region where optical loss in the fiber is minimized. Also, the erbium-doped fiber amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty.
The gain characteristics of a rare-earth doped optical amplifier depend on the dopants and co-dopants used to make the fiber core, the particular rare-earth ion employed, and the pumping mechanism that is used. Ideally, optical amplifiers should offer a high degree of gain stabilization to reduce power transients or excursions caused by the EDFA cross-saturation effect. Unfortunately, the gain of an optical amplifier may undergo substantial variations while amplifiying a signal. For example, when the individual WDM channels of a WDM communication system are randomly turned on and off (such as in adding or dropping a channel) the gain of the EDFA's could undergo undesirable fluctuations due to saturation effects.
To overcome this problem, gain clamped EDFA's have been developed which provide a stabilized gain. In a gain clamped EDFA, the gain of the EDFA is stabilized by spectrally selective optical feedback sufficient to cause it to lase at some predetermined wavelength separated from the waveband within which the amplifier is designed to function. An example of a gain clamped EDFA is disclosed in U.S. Pat. No. 5,872,649. One advantage of such EDFA's is that WDM transmission systems can be upgraded by adding more channels without adjusting the pump power or the gain shape of the EDFA's. Additionally, the use of gain-clamped EDFA's not only simplifies the upgrading process but also ensures that any power transients that arise in the remaining channels are reduced. Similarly, power transients may also arise when an optical fiber is inadvertently cut, and in such cases gain-clamped EDFA's also reduce the power transients.
A consequence of this form of gain control is that the compensating signal generated in the optical feedback loop may at times have a very high power level, thus making it susceptible to nonlinear interactions such as stimulated Brilloun scattering (SBS). This problem is discussed in Yu et al., "Stimulated Brillouin Scattering of the Compensating Signal in All-Optical Link-controlled Amplifier Systems."
Accordingly, it is desirable to provide a gain clamped optical amplifier in which the adverse effects of SBS are reduced.